Before starting a short-term test of less than four days, how long should the active system have been operating?

For a short-term radon test lasting less than four days, having the active system operate for at least 24 hours prior to the test is crucial for ensuring accurate measurement. This duration allows the system to stabilize and effectively mitigate any prior radon buildup in the environment. When an active radon measurement system is turned on, it needs time to pull air through the collection device and reach a consistent state where the radon levels can be accurately measured.

If the system runs for too short a period, such as a few minutes or even two hours, it may not effectively clear the space of any existing radon and could lead to inaccurate or misleading results. A week would be excessive prior to a short-term test, as it may not reflect the radon levels expected during the testing period. Thus, the 24-hour operation before starting the test strikes the right balance, allowing the system to function optimally while maintaining relevance to the short-term testing objective.